1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system which comprises, in addition to a master cylinder, a regulator operatively connected to the master cylinder for regulating a hydraulic power pressure supplied from a power source to a regulated hydraulic pressure in response to operation of the master cylinder and applying the regulated hydraulic pressure to wheel brake cylinders.
2. Description of the Prior Art
In conventional service braking systems for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting wheel brake cylinders mounted on road wheels with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken, braking operation is achieved by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuits system.
In order to reduce the force required to operate a brake pedal in braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compress air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such as the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal. It has been proposed to employ the hydraulic booster, in the hydraulic braking system, as a dynamic hydraulic braking pressure generator in addition to the master cylinder. In other, words a hydraulic pressure boosted by the booster (hereinafter referred to as boost pressure) in response to depression of the brake pedal is applied directly to a hydraulic circuit. For example, as shown in Japanese Patent Laid-open Publication No. Sho 57-172863 or Sho 59-227552, boost pressure of the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuits system in order to reduce the stroke of the brake pedal.
However, in this prior hydraulic braking system, since the hydraulic pressure in the hydraulic booster, i.e., boost pressure, is applied directly to the wheel brake cylinder in one hydraulic circuit, it is necessary to be adjusted with the hydraulic braking pressure generated from the master cylinder in the other hydraulic circuit in view of the distribution of the braking force applied to road wheels. At the same time, in this hydraulic booster, a servo operation to the master cylinder has to be achieved. Accordingly, there are many requisites in designing not only the hydraulic booster but also the master cylinder, so that it manufacturing cost becomes high.
In accelerating operation of the above hydraulic braking system, it is necessary to prevent a slip on driven wheels of the road wheels, for providing an optimum accelerating performance. For this purpose, a connecting valve is disposed in a hydraulic passage connecting the hydraulic braking pressure generator to the wheel brake cylinders provided on the driven wheels, at a position upstream of valve means for controlling the communication between the hydraulic pressure generator and the wheel brake cylinders and the communication between a reservoir and the wheel brake cylinders, as disclosed in U.S. Pat. No. 4,565,411.
According to the hydraulic braking system described above, the road wheels are prevented from locking in braking operation by suitably controlling the hydraulic braking pressure applied to the wheel brake cylinders by means of the valve means, and the slip of the driven wheels is prevented in starting or accelerating operation by means of the valve means and the connecting valve which normally interrupts the flow between the wheel brake cylinders of the driven wheels and a pressure accumulator, i.e. the power source.
However, in the above braking system, since the connecting valve is disposed at a position upstream of the valve means, it is difficult to install the connecting valve and arrange the necessary piping for providing the anti-slip operation in a braking system already equipped with the valve means for the anti-locking operation. Further, in the case where the driven wheels are rear road wheels, a hydraulic pressure from the power source is applied to the valve means through a regulator valve such as a proportioning valve so that the hydraulic pressure to be controlled by the valve means is decreased.